Abstract
Wild harvest of seaweed supports small-scale, high-value industries in a number of regions in the world. Information is lacking on how increasing carbon dioxide (CO
2
) concentrations in seawater could impact seaweeds in wild harvest situations. This study focuses on a mixed unattached loose-lying red algal community of
Furcellaria lumbricalis
in close association with
Coccotylus truncatus
that is found in the West Estonian Archipelago Sea, NE Baltic Sea. In Estonian coastal waters, the wild harvest of
F
.
lumbricalis
started in 1960s and it has since been used as raw material for furcellaran production. The aim of this study was to determine how ocean acidification may impact the balance of these two red algal species in the community. Mechanistic assessment of the carbon physiology of
F
.
lumbricalis
and
C
.
truncatus
was used to predict productivity and competitive interactions between these species in a high-CO
2
world. Carbon use strategies in macroalgae were determined by analysing the natural abundances of carbon isotopes (δ
13
C), pH drift experiments and photosynthesis vs. dissolved inorganic carbon (DIC) curves. Photosynthesis of
F
.
lumbricalis
(operating with a CO
2
concentrating mechanism or CCM) performed worse along the broader range of DIC concentrations compared to
C
.
truncatus
(non-CCM), especially those characterized under future climate conditions. Therefore, changing seawater carbon chemistry through ocean acidification has the potential to influence the balance of
F
.
lumbricalis
and
C
.
truncatus
in the community and the efficiency of the wild harvest of this community and the quality of product provided.